Depending on the application for which they are implanted in a patient, implantable medical devices (IMDs) may include a variety of electrical and/or mechanical components. Typically, an IMD includes a rigid housing that houses all of its components, which are generally fragile, to protect the components from forces to which they would otherwise be exposed when implanted within the human body. In order to avoid potentially harmful interactions between the components and bodily fluids, e.g., corrosion, IMD housings are typically hermetically sealed. Many IMD housings are fabricated from Titanium because of its desirable rigidity and biocompatibility. Components common to most IMDs include a hybrid circuit that includes digital circuits, e.g., integrated circuit chips and/or a microprocessor, and analog circuit components. Most IMDs also include a battery to provide power to the digital and analog circuit components.
Most IMDs rely on a non-rechargeable, e.g., primary, battery as a source of power. When a primary battery is no longer able to provide adequate power for an IMD, the IMD must be explanted, and either the battery or the entire IMD must be replaced. The “lifetime” of a primary battery depends on the power requirements of the IMD, and the amount of power stored by the battery. The amount of power stored by a battery is closely related to its size, while the amount of power required by an IMD is primarily dependent of the type of therapy delivered by the IMD. For example, implantable neurostimulators and implantable pumps consume power at a relatively higher rate than cardiac pacemakers and IMDs used primarily for patient monitoring. Consequently, designers of IMDs with high power requirements, such as implantable neurostimulators and implantable pumps, often must choose between use of an undesirably large primary battery, or potentially exposing the patient to the risks associated with an eventual surgical procedure to explant the current IMD and implant a new IMD.
In response to the problems associated with the use of primary batteries, some IMDs with high power consumption have been configured as “radio frequency” (RF) systems in which the IMD does not include an implanted power source, but instead receives power from an external power source via transcutaneous inductive energy transfer. Typically the external power source includes a rechargeable battery and is coupled to a primary (external) coil, and the IMD includes or is coupled to a secondary (implanted) coil. Because the IMD in such systems does not include an internal power source, the patient must always wear, or otherwise carry, the external power source, and must keep the primary coil proximate to and aligned with the secondary coil at all times. Further, the patient may periodically have their movement restricted while the external power source is recharged, e.g., via a wall receptacle. On the whole, patients may view such systems as burdensome and restricting.
Other IMDs have been configured to include a rechargeable power source, e.g., a rechargeable battery, within the IMD. Typically, the rechargeable power source is periodically recharged by an external power source, i.e., a recharging device, via transcutaneous inductive energy transfer. Rechargeable batteries used in IMDs may have a longer lifetime and/or a smaller size than primary batteries. Further, when an IMD uses a rechargeable battery, the patient is free to move without the recharging device between recharging sessions.
The effectiveness of the recharging sessions, e.g., the time required to recharge the rechargeable battery, is dependant upon a number of considerations, such as the amplitude and frequency of the current induced in the secondary coil. The amplitude of the current induced in the secondary coil is, in turn, dependent of the proximity and alignment of the primary and secondary coils. Conventional IMD recharging devices may make it difficult for the patient to properly align the primary coil with the secondary coil, and/or to maintain proper alignment during a recharging session. Patients may have particular difficulty achieving and maintaining proper alignment for recharging where their IMD is implanted on or within their cranium. In particular, patients may have difficulty properly aligning the coils on a part of the body which they can see, if at all, only with the aid of a mirror. Further, traditional means for maintaining alignment of the coils during a recharge session, e.g., an adhesive patch carrying the primary coil, may be ineffective in cases in which the IMD is located beneath the patient's hair.